1. Field of the Invention
The present invention relates to a L-lactic acid polymer composition, and a molded product and a film of the composition. More particularly, the invention relates to a molded product and film having excellent transparency and flexibility.
2. Description of the Related Art
Resins having excellent transparency and flexibility are generally prepared by addition of plasticizers or elastomers to these resins. Addition of the plasticizers, however, leads to bleeding of the plasticizers from molded articles and is liable to cause blocking of the molded articles. A process has also been known to use the plasticizers in combination with an inorganic material as a blocking inhibitor. However, the inorganic material tends to decrease transparency of the molded articles. Further, addition of the elastomers is effective for improving flexibility of the molded products. However, the elastomers are liable to make the molded articles opaque.
Resins such as non-rigid polyvinyl chloride and specific polyolefin have been known to have excellent transparency and flexibility. These resins, however, increase the amount of scrap on waste disposal. Additionally, these resins scarcely degrade in the natural environment and semipermanently remain under the ground even in the case of burial disposal. Further, abandoned plastics have led to problems of impairing the aesthetics of the enviroment and destructing the living environment of marine organisms.
On the other hand, a homopolymer of lactic acid, a copolymer of L-lactic acid with D,L-lactic acid and a copolymer of lactic acid with other hydroxycarboxylic acids (hereinafter referred to generally as L-lactic acid polymer) have been developed as polymers which are thermoplastic and biodegradable. These polymers can be completely biodegraded in an animal body within a few months to an year. These polymers initiate degradation within a few weeks under humid environment, for example, in the case of being left in soil or sea water and disappear in one to several years. Further, these plastics have been characterized in that decomposition products are lactic acid, carbon dioxide and water which are non-toxic for the human body.
Lactic acid which is used as a raw material of polylactic acid has been prepared by a fermentation process or chemical synthesis. L-lactic acid, in particular, has been manufactured in a large scale by the fermentation process and its price has been reduced. Additionally, L-lactic acid has a feature of providing high stiffness for the resulting polymer. Consequently, various uses of L-lactic acid polymer are now under development.
Molded products prepared by injection molding, extrusion, and blow molding and films of L-lactic acid polymer are excellent in stiffness. These products, however, have low flexibility and have been scarcely applied to uses which require flexibility. DE Pat. 3,635,679 has disclosed addition of acetyl tributyl citrate to a surgical monofilament of L-lactic acid polymer as a plasticizer for improving flexibility. The patent, however, has no description on a stretched film having transparency, flexibility or heat resistance. Further, the surgical monofilament having flexibility can be certainly prepared by the process of a patent. The process, however, cannot be generally applied to the production of common articles and films because of insufficient processability. That is, the process of the patent leads to blocking in the course of molding operation, extrusion is liable to become irregular and the resulting films are practically inferior because of a poor slipping property.
U.S. Pat. No. 5,076,983 has disclosed a film comprising 0.1-8% by weight of a lactide monomer as a plasticizer, When the polymer is cast on a drum to form a film in the process, lactide separates from the film depending upon the concentration of lactide, sticks on the roll surface and thus leads to contamination of the drum surface and irregular thickness of the resultant film. As a result, the amount of addition is limited and the film exhibits an elongation of 30-140% as the effect of addition, according to the description. When a polymer is used in the form of a film, properties generally required for the film are an elongation higher than the above range, wind-up property in the processing step and slipping property under conditions of use. The films obtained by the above processes are practically still inferior in view of these respects.
It has been known to add silica as an inorganic filler to a biodegradable polymer. Exemplary patents for adding such an additive to the biodegradable polymer include Japanese Laid-Open Patent HEI 5-70,696, Japanese Kohyo HEI 4-504731 and Japanese Kohyo HEI 5-508,669.
Japanese Laid-Open Patent HEI 5-70696 has disclosed that 10-40% by weight of calcium carbonate or hydrated magnesium silicate (talc) having an average particle size of 20 .mu.m or less are added to biodegradable plastics such as a 3-hydroxybutyrate/3-hydroxypivalate copolymer, polycaprolactone and polylactic acid in the preparation of a raw material of plastic containers, and the object is to accelerate decomposition of the biodegradable plastics after waste disposal by addition of a large amount of inorganic fillers.
Japanese Kohyo HEI 4-504731 (WO 90/01521) has described that properties such as hardness, strength and temperature resistance can be varied by addition inorganic fillers such as silica and kaolinite to a thermoplastic lactide plastic. Japanese Kohyo HEI 5-508,669 (WO 92/01737) has disclosed the addition of calcium silicate or talc as a processing aid to polylactic acid when it is used as a biodegradable foam material. The processing aid is used as a nucleating agent in the foaming extrusion process. However, any of these cases does not improve anti-blocking properties between the films.